The present invention relates to an apparatus for supplying a working fluid in an electric discharge machine, and more particularly to an apparatus of this kind, which is capable of linearly controlling the supply of the working fluid.
In an electric discharge machine, a working region is supplied with a working fluid such as water for cooling, keeping electric insulation for electric machining, and discharging machining chips. For instance, in a wire-cut electric discharge machine, the working fluid is supplied through an injection nozzle to a gap between a wire electrode and a workpiece. As an amount or pressure of supply of the working fluid is increased, the machining chips are easily discharged and hence efficiency in electric discharge machining is increased so that machining speed is improved. On the other hand, an increased working fluid pressure causes the wire electrode to vibrate and a turbulent flow, accompanied with bubbles, occurs and results in the wire electrode being cut off. Accordingly, it is necessary to control the working fluid amount to a desired value, preferably in a linear manner.
To this end, conventionally, in a wire-cut electric discharge machine of a type supplying the working fluid from a working fluid tank to an injection nozzle, the flow rate of the working fluid is controlled by controlling the rotational speed of a motor of a variable displacement type pump provided between the tank and the nozzle by means of invertor control. However, according to invertor control, a pulsation is found in the motor rotation and hence in the flow of the working fluid at low motor speeds. Further, invertor control has a restriction relating to power supply voltage. Moreover, a surge current or voltage can be generated in invertor control wherein an electric current or voltage applied to the motor is intermittently turned on and off, resulting in damaging to a surge absorber provided for protection of a control system of the electric discharge machine.